1. Field of the Invention
This invention relates to a ball end mill.
2. Description of the Related Art
In recent years, there have been strong demands for higher precision die molding. In response, much research and development has been conducted in relation to more precise and longer-lived cutting tools. One of these, a ball end mill, performs extremely high-precision half-spherical cutting, and is needed in cutting of the curved surface, particularly in finishing processes.
A conventional ball end mill comprises two approximately quarter-circular cutting blades, which are provided opposite each other at the front tip along the diameter; the cutting blades touch at a point on a rotational axis of the end mill, or alternatively, they are given deviating rotational axes, so as to form a chisel section between the cutting blades near the rotational axis of the end mill.
In the ball end mill disclosed in Japanese Patent Application, First Publication No. 2002-52412, approximately circular cutting blades are provided continuously through the rotational axis at the tip of a tool main body, which rotates around the rotational axis; a rake face and a flank are provided continuously on both sides along each cutting blade, the rake face and the flank facing each other with the cutting blades therebetween. The rake angle and clearance angle change continuously on both faces on either side of the cutting blades.
However, the conventional ball end mill described above has problems such as the following.
In a ball end mill having two cutting blades touching at a point on the rotational axis, the contacting parts of the cutting blades have poor rigidity, making the cutting blades and the tool liable to break and shortening the life of the tool.
Providing a chisel section near the rotational axis reduces breakage of the cutting blades and the tool, however, since the cutting force in the chisel section is insufficient, the cutting blades move sideways and pressing against a work material. Consequently, tears and the like occur in the chisel section, making it impossible to obtain a smooth cutting surface having low surface roughness. As a result, this type of ball end mill has a drawback that it cannot be used in work requiring high precision, such as finishing.
In the end mill disclosed in the above Japanese patent application, the cutting blades are continuous, solving the problems mentioned above; however, since the circumferential speed of the cutting blades on the rotational axis is zero, the cutting resistance increases near the rotational axis, and as a result, abrasion of the cutting blades is severe, leading to breakage and shortening the life of the tool.
Furthermore, the ball end mill described in the above Japanese patent application has another drawback, namely, that the rake face and flank must be made into complex concaved surfaces, increasing the manufacturing cost. This work is particularly difficult when using highly rigid material which is essential for highly precise cutting, and makes the manufacturing cost extremely expensive.